fix(local-store): never save virtual flag manifests (__unread__ etc.)

The list view's manifest_dirty path was missing the !is_virtual_flags
guard, so browsing the Unread/Flagged virtual views caused the synthetic
in-memory manifest to be written to __unread__.tsv / __flagged__.tsv.
These stale files then fed back into local_flag_search on the next open,
inflating counts and attributing messages to the wrong folder.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: csjpeter

libemail/src/domain/email_service.c

@@ -2767,7 +2767,7 @@ int email_service_list(const Config *cfg, EmailListOpts *opts) {
             printf("\n");
             fflush(stdout); /* show row immediately as it arrives */
         }
-        if (manifest_dirty && !is_virtual_search) manifest_save(folder, manifest);
+        if (manifest_dirty && !is_virtual_flags && !is_virtual_search) manifest_save(folder, manifest);
         if (load_interrupted) goto list_done;
 
         if (!opts->pager) {